Adding php modules

I am trying to update my phplist program using the phplist updater that I have used many times in the past. This time it is telling me a php module is missing. the exact error message is

“The following php modules are required. Please install them to continue.

Could someone tell me where to find the file and where I should place it so the updater will work again? I would like to use the new version but I am not teh best at finding how to fix things when they do not work right.

Thanks in advance